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Sulfates plus black carbon a nasty combo for warming

Fossil fuels may be doing an even more efficient job of warming the planet …

Fossil fuels may be doing an even more efficient job of warming the planet than we thought. A new study shows that black carbon generated by fossil fuels seems to warm the planet more than other sources, and sulfates generated by burning cheaper fuels help the black carbon absorb even more energy. The scientists who authored the paper note that policies will likely need to tamp down on both substances in order to make a dent in the rate of global warming.

Researchers have long considered black carbon a culprit behind the increasingly warm atmosphere, as it holds incoming heat rather than reflecting it. They haven't been able to decisively point fingers, though, because it's difficult to pinpoint how much warming the black carbon causes. The source of the carbon seemed to play an important role, as did the ambient amount of other substances, like sulfates.

To test the effects of pollution content and origin on its ability to absorb heat, a group of researchers tested a few different plumes of burnt fuel in East Asia. The plumes originated from locations like Shanghai and Beijing; a few came from less-centralized land areas and the ocean.

The researchers compared their findings to data that had been collected from the same plumes in 2008, as part of testing prior to the 2008 Olympics. The tests used both ground level and unmanned aerial vehicles to collect samples, creating a vertical profile of the plumes' contents over time.

When they analyzed their findings, the researchers noticed first that plumes generated mostly from fossil fuels, like coal and petroleum, had higher heat absorption rates than those created by burning fuels like wood. They saw the highest absorption in the plume originating from Beijing, where fossil fuel use is very heavy.

The plumes over the ocean came from ships using low-quality fuels that generate a large amount of sulfate when burned. These samples allowed the authors to see that plumes with higher ratios of sulfate to black carbon tended to absorb more heat as well.

Sulfate, unlike black carbon, reflects incoming warmth rather than absorbing it. On its own, sulfate could help cool the atmosphere, but when mixed with black carbon, it acts as a net for all the warmth that would otherwise pass through. A plume with lots of sulfate appears to give the surrounding black carbon a second chance to absorb that heat.

The most striking find was that even when the researchers controlled for the black-carbon-to-sulfate ratio, the plumes originating from fossil fuel still absorbed heat twice as effectively as the biomass plumes. As the proportion of black carbon created from fossil fuel has increased three-fold over the last century, many of the pollution plumes in the world have turned into heat-absorbing machines.

Some efforts, though, have already shown promise: the researchers point out that emission controls that went into effect in 2008 have reduced the emission rate of black carbon by about 25 percent, and reduced the ratio of sulfate to black carbon by about 20 percent.

The authors note that efforts to reduce sulfate emissions are already underway, though focusing on decreasing the ratio of sulfate to black carbon would be more effective. They also advocate for emissions controls to place particular focus on the restriction of fossil fuels, as the resulting black carbon absorbs heat too efficiently for its own good.

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Casey Johnston
Casey Johnston is the former Culture Editor at Ars Technica, and now does the occasional freelance story. She graduated from Columbia University with a degree in Applied Physics. Twitter@caseyjohnston